The patent application of Gerald R. Reafler Ser. No. 116,426 filed Nov. 3, 1987 describes a new kind of protective and decorative sheet material which can be thermoformed and bonded to the surfaces of automobile panels, household appliances and other articles. The sheet material is formed by the continuous laminar flow coating of a paint, such as a water-based polyurethane composition, onto a moving web of a stretchable polymeric film onto which an optional primer or tie coat has previously been coated. Then, over the dried paint layer, is coated a clear top coat to provide gloss and other desired surface qualities.
A principal way of using this sheet material is to bond it adhesively to automobile panels or the like by thermoforming. To provide this capability the stretchable web, which on one side has a dried paint layer and top coat, is coated on its other side with a pressure-sensitive adhesive. After coating and drying the adhesive layer, the coated web must be wound up as a supply roll for future use in thermoforming. The kind of adhesive that is desired for bonding the sheet material to surfaces, however, is so tenaciously sticky that a protective film must be laminated to the adhesive-coated side to facilitate handling and later unwinding of the roll.
Although the lamination of protective release sheets to adhesive-coated webs is known (see, for example, U.S. Pat. Nos. 3,361,609, 4,693,858 and 4,751,121) the kind of web with which the present invention is concerned presents unusual problems that are not solved by conventional interleaf laminating practices.
A number of polymeric films, such as polyethylene terephthalate films, are useful as the releasable protective web. They adhere adequately to the pressure-sensitive adhesive, and can later be peeled away. It has been found, however, that when the laminated web is rolled up and later unwound, the releasable protective film partially delaminates from the adhesive layer. This can occur immediately upon unwinding or when the web is laid flat to be cut into sheets for thermoforming. This is not usually a complete delamination but a partial delamination which appears as wrinkles or linear delaminations in small or large areas of the web. The wrinkles can create a visible pattern in the viewing side of the sheet material. For high quality product the wrinkled area must be cut out or trimmed from the edges of the sheet material, thus causing substantial waste.
It appears that the wrinkling problem is related to the fact that the paint coated web is stretchable and thermoplastic, as it must be in order that it can be thermoformed and bonded to three-dimensional substrates. On the other hand, since the web must be drawn under tension through heated drying zones during its manufacture, it appears that at least a small degree of undesired stretching of the web occurs before the protective web is laminated to it.
Although the invention should not be limited to this theoretical explanation of the mechanism it appears that the slightly stretched paint coated web shrinks after it has been laminated to the release film. When wound as the outer layer of a roll, the web remains under tension and little if any delamination occurs but when unwound and flattened the wrinkles appear. A solution to this problem has been needed.